is_subclass_of() document fix

gistfile1.diff

Index: is-subclass-of.xml
===================================================================
--- is-subclass-of.xml	(リビジョン 324628)
+++ is-subclass-of.xml	(作業コピー)
@@ -42,7 +42,8 @@
      <term><parameter>allow_string</parameter></term>
      <listitem>
       <para>
-       Whether to call autoloader if the class doesn't exist.
+       If this parameter set to false, string class name as <parameter>object</parameter>
+       is not allowed. This also prevents from calling autoloader if the class doesn't exist.
       </para>
      </listitem>
     </varlistentry>